This invention relates generally to implantable medical devices (IMDs) having sense amplifiers for sensing physiologic signals and parameters, RF telemetry capabilities for uplink transmitting patient data and downlink receiving programming and interrogation commands to and from an external programmer or other medical device, and other electronic circuitry susceptible to electromagnetic interference (EMI), and more particularly to miniaturized, integrated circuit notch filters for use in such circuitry to reject EMI.
A wide variety of IMDs that employ electronic circuitry for providing electrical stimulation of body tissue and/or monitoring a physiologic condition are known in the art. A number of IMDs of various types are known in the art for delivering electrical stimulating pulses to selected body tissue and typically comprise an implantable pulse generator (IPG) for generating the stimulating pulses under prescribed conditions and at least one lead bearing a stimulation electrode for delivering the stimulating pulses to the selected tissue. For example, cardiac pacemakers and implantable cardioverter-defibrillators (ICDs) have been developed for maintaining a desired heart rate during episodes of bradycardia or for applying cardioversion or defibrillation therapies to the heart upon detection of serious arrhythmias. Other nerve, brain, muscle and organ tissue stimulating medical devices are also known for treating a variety of conditions.
There are at least four paramount considerations or goals that are taken into account in the design and fabrication of IMDs. First, the IMD operation must be safe, reliable and effective in delivering a therapy and/or monitoring a physiologic condition. Second, the IMD must be long lived and be cost-effective relative to alternative therapies. Third, the IMD must be reasonably miniaturized so that it can be implanted without being uncomfortable and cosmetically distressing to the patient. Finally, each new generation of IMD must satisfy the first three considerations while providing an ever increasing number of performance features and functions that are clinically beneficial to the patient and useful to the medical community.
For example, over the past 20 years, ICD IPGs have evolved, as described in some detail in commonly assigned U.S. Pat. No. 5,265,588, from relatively bulky, crude, and short-lived ICD IPGs simply providing high energy defibrillation shocks to complex, long-lived, and miniaturized ICD IPGs providing a wide variety of pacing, cardioversion and defibrillation therapies. Numerous improvements have been made in cardioversion/defibrillation leads and electrodes that have enabled the cardioversion/defibrillation energy to be precisely delivered about selected upper and lower heart chambers and thereby dramatically reducing the delivered shock energy required to cardiovert or defibrillate the heart chamber. Moreover, the high voltage output circuitry has been improved in many respects to provide monophasic, biphasic, or multiphase cardioversion/defibrillation shock or pulse waveforms that are efficacious, sometimes with particular combinations of cardioversion/defibrillation electrodes, in lowering the required shock energy to cardiovert or defibrillate the heart.
The first implanted automatic implantable defibrillator (AID) IPG housing disclosed in U.S. Pat. No. 4,254,775 was very large and had to be implanted in a patient""s abdominal region. Since that time, the ICD IPGs have been reduced in size while their complexity has been vastly increased. Battery energy requirements for powering both the low voltage integrated circuits (ICs) and for providing the cardioversion/defibrillation shocks have been reduced while battery energy density has been increased and battery configuration made more conforming to the interior space of the ICD IPG housing. Miniaturized, flat high voltage output capacitors that can be shaped to fit the allocated housing space and miniaturized high voltage switching components have been developed and employed. All of these improvements, together with the above-mentioned cardioversion/defibrillation improvements have contributed to a significant reduction in the volume of the ICD IPG housing without sacrificing longevity and capabilities. Similar improvements in reducing housing volume have been made in other IMD IPGs, particularly implantable cardiac pacemakers, nerve stimulators and monitors, over the same time period.
Such ICDs as well as implantable cardiac pacemakers and implantable cardiac monitors have been clinically employed or proposed that include capabilities of sensing one or more physiologic parameter and triggering delivery of a therapy and/or storage of physiologic data. With the exception of certain subcutaneously implanted monitors, all such IMDs comprise a hermetically sealed IPG housing containing the battery and electronic circuitry that is coupled through an elongated lead to a remote sense/stimulation electrode or physiologic sensor located typically in a heart chamber, heart muscle or blood vessel. Such physiologic parameters include electrical heart signals and other physiologic parameters, e.g. blood pressure at various locations of the heart and vascular system, respiration, blood temperature, blood pH, and blood gas concentrations.
The sensing of these physiologic parameters involves the detection of minute electrical signals in an inherently electrically noisy environment including both ambient EMI in the patient""s environment as well as electrical signals either generated in the body or in the sensor due to patient motion or respiration or the like. Sources of EMI include metal detectors such as are used in airports, welders, radio transmitters (broadcast and two-way), cellular phones, microwave ovens, electronic article surveillance (EAS) systems, etc. The electrical signals conducted to the implanted device from the electrodes implanted in the heart may thus include EMI superimposed on the heart""s natural cardiac signal. In dual chamber cardiac pacing and ICDs, it is necessary to sense and discriminate between P-waves and R-waves despite the presence of EMI and various pathologic cardiac conditions that can cause the sense amplifier to oversense or undersense these particular signals. It is currently the practice to start blanking periods upon delivery of a pacing pulse in one heart chamber to avoid saturating the sense amplifier circuitry and refractory periods that are used to disregard sense events representing artifacts of the induced depolarization or a closely timed depolarization in another heart chamber. To avoid sensing EMI, it has been the practice to low pass filter the sense amplifier input and to programmably adjust the sensitivity of the sense amplifier to a level that renders it insensitive to low level EMI but capable of sensing the peak voltages of the signal of interest. Descriptions of various sensing windows and noise rejection techniques for implantable and external pacemakers that have been employed or proposed are set forth in U.S. Pat. Nos. 4,357,943, 4,390,020, 4,401,119, 4,436,093, 4,596,292, and 5,188,117.
Although the IMD sense amplifiers are filtered to attenuate noise superimposed on a cardiac signal, in some situations the noise component may be such that the filters cannot adequately eliminate the noise. If a patient with an IMD walks through a metal detector, for example, the resulting EMI signal may overwhelm the cardiac signal picked up by the electrodes. Although the IMD may be able to determine that it is receiving an excessive amount of noise, the IMD may be unable to extract the true cardiac signal from the noise. Because the true cardiac electrical signal cannot be accurately ascertained, the IMD""s operating system cannot determine when the vulnerable period of each cardiac cycle is occurring. Such implantable cardiac pacemakers and ICDs having a pacing capability are therefore often supplied with a xe2x80x9creversion modexe2x80x9d of operation that will cause no harm to the patient. However, the reversion mode also cannot provide the optimum pacing therapy that the patient may require at that very same time, and patient safety or comfort may be compromised. In the context of an ICD, the inability to distinguish high level EMI from a malignant tachyarrhythmia could either cause a mistaken delivery of a cardioversion/defibrillation shock or inhibit delivery of a warranted cardioversion/defibrillation shock.
The expansion of wireless communication modes occupying more and more frequency bands will continue to cause new EMI caused oversensing, undersensing, and inhibition problems to develop. It is expected that potential EMI frequencies will include those used by cellular phones, wireless phones, mobile radio, amateur (ham) radio, radar, ISM, wireless LAN, Bluetooth, HomeRF, broadcast AM, FM, UHF, and VHF signals, and MRI scans, etc. Interference sources and frequency bands employed will vary from country to country. See for example, xe2x80x9cInfluence of D-Net (European GSM-Standard) Cellular Phones on Pacemaker Function in 50 Patients with Permanent Pacemakersxe2x80x9d, by Wilke et al. (PACE, vol. 19, pp. 1456-8, October 1996) and xe2x80x9cPacemaker Inhibition and Asystole in a Pacemaker Dependent Patientxe2x80x9d, by Yesil et al. (PACE, vol. 18, p. 1963, October 1995). Potential EMI sources transmit in the 820-896 MHz, 1.93-1.99 GHz, and 2.402-2.48 GHz bands.
Therefore, there remains a need to improve the ability to sense low level cardiac signals in the presence of high level EMI in order to optimize IMD operation during EMI.
Similar problems can be caused by high level EMI that is superimposed onto lead conductors extending between remote physiologic sensors and the signal processing circuitry within the IMD housing. The lead conductors are used to conduct power to remote sensors or transducers and to conduct physiologic signals developed by the powered sensors or transducers back to the signal processing circuitry. Commonly assigned U.S. Pat. No. 5,535,752 describes such a combination of a pressure and temperature sensing lead and circuitry for powering the remote sensors or transducers and processing their signals. High level EMI signals superimposed on the physiologic signals can introduce errors into the signal processing, resulting in erroneous stored physiologic data or an inappropriate therapy delivery.
Therefore, there remains a need to improve the ability to sense low level physiologic sensor signals in the presence of high level EMI in order to optimize IMD operation during EMI.
In addition, such EMI can disrupt the programming or interrogation of an IMD. An ever increasing variety of programmable operating modes and monitoring and physiologic data collection capabilities have been incorporated into IMDs. Remote programming and interrogation of IMD operating modes and parameters have been implemented in the above-described IMDs employing uplink (from the IMD) and downlink (to the IMD) telemetry transmissions between an RF transceiver within the IMD and an external transceiver of an external xe2x80x9cprogrammerxe2x80x9d. Such programmers are used to program the IMD by downlink telemetry transmission of commands that are received and stored in memory incorporated within the IMD that change an operating mode or parameter value governing a function performed by the IMD.
Both non-physiologic and physiologic data (collectively referred to herein as xe2x80x9cpatient dataxe2x80x9d) can be transmitted by uplink RF telemetry from the IMD to the external programmer or through the patient""s body to another IMD. The physiologic data typically includes stored and real time sampled physiologic signals, e.g., intracardiac electrocardiogram amplitude values, and sensor output signals. The non-physiologic patient data includes currently programmed device operating modes and parameter values, battery condition, device ID, patient ID, implantation dates, device programming history, real time event markers, and the like. In the context of implantable pacemakers and ICDs, such patient data includes programmed sense amplifier sensitivity, pacing or cardioversion pulse amplitude, energy, and pulse width, pacing or cardioversion lead impedance, and accumulated statistics related to device performance, e.g., data related to detected arrhythmia episodes and applied therapies.
The RF telemetry transmission system that evolved into current common usage relies upon magnetic field coupling through the patient""s skin of an IMD IPG antenna with a closely spaced programmer antenna. Low amplitude magnetic fields are generated by current oscillating in an LC circuit of an RF telemetry antenna of the IMD or programmer in a transmitting mode. The currents induced in the closely spaced RF telemetry antenna of the programmer or IMD are detected and decoded in a receiving mode. Short duration bursts of the carrier frequency are transmitted in a variety of telemetry transmission formats. In the MEDTRONIC(copyright) product line, the RF carrier frequency is set at 175 kHz, and the IMD RF telemetry antenna located within the IMD housing is typically formed of coiled wire wound about a bulky ferrite core.
There are a number of limitations in the current MEDTRONIC(copyright) telemetry system employing the 175 kHz carrier frequency. First, using a ferrite core, wire coil, RF telemetry antenna results in a very low radiation efficiency because of feed impedance mismatch and ohmic losses and a magnetic field strength attenuated proportional to distance. These characteristics require that the implantable medical device be implanted just under the patient""s skin and preferably oriented with the RF telemetry antenna closest to the patient""s skin so that magnetic field coupling is provided. To ensure that the data transfer is reliable, it is necessary for the patient to remain still and for the medical professional to steadily hold the RF programmer head against the patient""s skin over the IMD for the duration of the transmission. The time delays between downlink telemetry transmissions depend upon the user of the programmer, and there is a chance that the programmer head will not be held steady. If the uplink telemetry transmission link is interrupted by a gross movement, it is necessary to restart and repeat the uplink telemetry transmission.
In addition, it is necessary to insure that the IMD is not inadvertently mis-programmed by EMI that the patient is exposed to that could mimic the modulated 175 kHz carrier frequency. The classic manner of preventing the reception of such signals has been to disable the telemetry transceiver unless a magnetically sensitive reed switch within the IMD housing is switched open or closed by an externally applied permanent magnet field held over the IMD. Moreover, the application of the magnet traditionally only takes place during programming and interrogation telemetry sessions initiated in the presence of a medical professional. In certain IMDs, the patient is provided with a magnet that he or she can apply when experiencing symptoms to trigger delivery of a therapy and/or storage of physiologic data.
The RF telemetry data transmission rate is limited employing a 175 kHz carrier frequency. As device operating and monitoring capabilities multiply, it is desirable to be able to transmit out ever increasing volumes of data in real time or in as short a transmission time as possible with high reliability and immunity to spurious noise. Also, in an uplink telemetry transmission from an IMD, it is desirable to limit the current drain from the implanted battery as much as possible, simply to prolong IMD longevity.
As a result of these considerations, IMD-programmer RF telemetry schemes have been proposed having the objectives of conserving space within the IMD housing, eliminating the need for the close, steady magnetic field coupling to enable uplink and downlink telemetry transmission at a greater distance, increasing the data transmission rate, and minimizing battery consumption. The uses of the 204-216 MHz band and the 902-908 MHz ISM band have been proposed in U.S. Pat. Nos. 5,944,659 and 5,767,791 for telemetry transmissions between externally worn patient monitors and fixed location networked transceivers for ambulatory patient monitoring in a hospital context. The frequency bands that will be available for high frequency telemetry transmissions with IMDs can vary from country to country, and the sources and bandwidths of EMI can also vary from country to country.
The majority of front end high Q band pass filters used in radio frequency (RF) and intermediate frequency (IF) stages of heterodyning transceivers use off-chip, mechanically resonant components such as crystal filters or surface acoustic wave (SAW) signal transmission systems. These greatly outperform comparable devices using transistor technologies in terms of insertion loss, percent bandwidth, and achievable rejection of noise signals. In SAW resonators, signal processing can be combined with spread spectrum filtering for low transmission power and high rejection against jamming and interference. Advantages are high sensitivity, high reliability, and a moderate size of 1 cm3.
Off-chip components are required to interface with integrated components at the board level, which constitutes an important bottleneck to miniaturization and the performance of heterodyning transceivers. Recent attempts to achieve single chip transceivers have used direct conversion architectures, rather than heterodyning and have suffered in overall performance. The continued growth of micromachining technologies, which yield highxe2x80x94Q on-chip vibrating mechanical resonators now make miniaturized, single-chip heterodyning transceivers possible. Microelectromechanical systems (MEMS) resonators yield ultra high Qs of over 80,000 under vacuum and center frequency temperature coefficients less than 10 ppm/xc2x0 C. and serve well as a substitute for crystal filters and SAW devices in a variety of high-Q oscillator and filtering applications. MEMS resonators are capable of frequency operation to GHz levels and filtering operation up to the 6th order.
There remains a need to block such sources of EMI from contaminating or jamming uplink and downlink telemetry transmissions employing such high frequency RF operating channels.
The use of low pass, band pass and high pass filters remains desirable to pass desirable signals and to block or attenuate EMI. Ideally, EMI blocking filters employed in IMDs would be passive, low power consuming, and small in size. One approach proposed more recently for low pass filters involves the incorporation of discrete chip or discoidal capacitors with feedthroughs extending through the hermetically sealed housing wall, as disclosed in commonly assigned U.S. Pat. No. 5,836,992. Such filter capacitors are electrically connected between the feedthrough pins that couple the circuitry of the IMD with the lead conductors and externally disposed telemetry antenna and the feedthrough ferrules attached to the housing. The capacitor values are chosen to block frequencies in excess of 1 MHz.
It would be more desirable to provide band pass filtering to reject or attenuate signal frequencies above and below the characteristic frequencies of a signal of interest. The use of narrow band pass notch filters formed of passive components in external or implantable pacemakers to block EMI but to pass low frequency heart P-waves and R-waves has been suggested but not employed in the above-referenced ""093 and ""117 patents. It is observed therein that notch filters formed of discrete inductors and capacitors into passive analog circuits are unsuitable for such uses due to circuit complexity, difficulty in controlling component tolerances, component drift with temperature and aging, and high current drain. In the context of an IMD, assembling complex notch filters from bulky discrete inductors also consumes valuable space within the limited confines of the IMD housing. Accordingly, the use of active notch filters tuned specifically to pass a narrow frequency bandwidth and to block higher and lower frequency signals is proposed in the above-referenced ""093 patent for an external cardiac pacemaker, where space is readily available and current consumption is less critical, since the battery is readily replaceable when spent.
In the ""117 patent, it is also suggested that the formation of a suitable notch filter from integrated circuit (IC) components is not feasible. Instead, use of a delta modulator and band pass filter to digitize the signal from the sense amplifier initial stage and then digitally notch filter the digitized samples to remove any EMI artifacts. This approach does not work well in an IMD because the high frequency clock rate that would be required to sample and digitize the signal to remove high frequency EMI would consume battery current at an unacceptable rate and shorten battery life.
In order to fit within a minimal space, all electronic circuits of the IMD circuitry are preferably formed as ICs, but implementation of RF telemetry circuitry has required use of discrete capacitors and inductors mounted along with the RF IC to a circuit board. The number of such discrete inductors and capacitors is not reduced, but is actually increased, in the above-described high frequency, high data transmission rate telemetry transceiver circuitry. The relocation of the bulky RF telemetry antenna to a less critical area outside the hermetically sealed IMD housing is necessary to operate in a high frequency RF telemetry bandwidth and reduces space requirements. However, the discrete inductor and capacitor components mounted with one or more IC chip to an RF IC module substrate render the RF module unduly large such that it occupies a substantial portion of the volume within the IMD housing.
The present invention is directed to an IMD having a hermetically sealed chamber defined by a hermetically sealed housing, wherein the housing has an inner and an outer wall surface of a predetermined contour and enclosing a housing cavity or chamber of a predetermined volume. In accordance with a basic aspect of the present invention, the circuitry of the IMD comprises at least one IC chip mounted on a substrate and a plurality of micro-machined IC fabricated inductors and capacitors combined to form a notch filter or IC fabricated MEMS notch filter structures that reduces the space occupied within the housing. They can be tuned to block passage of particular EMI frequencies, and a plurality of the notch filter circuits can be combined to block a plurality of such EMI frequencies.
The miniaturized notch filters make the IMD operation safer, more reliable and more effective in delivering a therapy and/or monitoring a physiologic condition by blocking the EMI frequencies. Secondly, the miniaturized, passive notch filters do not increase current consumption and can be fitted within the hermetically sealed housing, thereby enabling inclusion of additional device functions that are clinically beneficial to the patient and useful to the medical community. The space within the IMD housing can be occupied by other components or can result in making the housing itself smaller and possibly thinner in profile than it would otherwise be.
In a first aspect of the invention, IC fabricated inductors are integrated into one or more IC chips mounted to circuit module substrates. In a further aspect of the invention, discrete capacitors are surface mounted over IC chips to reduce space taken up on the module substrate and to shorten the conductive paths between the IC and the capacitor. In a still further aspect of the invention, each IC chip is mounted into a well of the module substrate and short conductors are employed to electrically connect bond pads of the module substrate and the IC chip.
The inductors are preferably fabricated as planar spiral wound conductive traces formed of high conductive metals to reduce trace height and width while maintaining low resistance, thereby reducing parasitic capacitances between adjacent trace side walls and with a ground plane of the IC chip. The spiral winding preferably is square or rectangular, but having truncated turns to eliminate 90xc2x0 angles that cause point-to-point parasitic capacitances.
The planar spiral wound conductive traces are further preferably suspended over the ground plane of the RF module substrate by micromachining underlying substrate material away to thereby reduce parasitic capacitances.
This summary of the invention and the objects, advantages and features thereof have been presented here simply to point out some of the ways that the invention overcomes difficulties presented in the prior art and to distinguish the invention from the prior art and is not intended to operate in any manner as a limitation on the interpretation of claims that are presented initially in the patent application and that are ultimately granted.